The invention relates to a printing machine for printing a substrate web, which printing machine comprises at least one printing module, such a printing module being provided with an impression roller and ink application means and a cylindrical element (also know as a printing cylinder) a cylindrical element, which cylindrical element (printing cylinder) extends parallel to the impression roller and abuts against the impression roller at a contact line with interposition of the substrate web, the cylindrical element (printing cylinder) being arranged to apply a desirable ink printing pattern to the substrate web.
Such an apparatus is known from practice. The known apparatus has the drawback that it is very time-consuming to change printing techniques in a printing module. When changing from, e.g., flexographic printing to silk-screen printing, the driving elements and the bearing elements of the ink application means must be exchanged and reset. Moreover, it is often necessary to exchange the impression roller. In general, for flexographic printing and letterpress printing another impression roller is actually used than for silk-screen printing. Not only is it time-consuming to exchange the impression roller, but, moreover, the substrate web must be removed for that purpose, which leads to a considerable amount of waste. The operational costs of the known apparatus are therefore high. Furthermore, the ink application means of different types require bearing and driving means of their own, which, upon purchase of the printing machine, lead to a very high investment. The purchasing costs of the apparatus known from practice are therefore very high as well.
The object of the invention is to provide a printing machine without the above-described disadvantages.
To that end, a printing machine of the type described in the opening paragraph is characterized according to the invention in that a relevant printing module is provided with a circumferential bearing which engages an outer surface of a relevant cylindrical element (printing cylinder), the or each circumferential bearing in an operating condition of the printing module being in a locking position in which the cylindrical element is pressed towards and against the impression roller, which circumferential bearing can be brought into a condition of exchange such that the cylindrical element can be taken from the printing module, the circumferential bearing being arranged to receive cylindrical elements intended for different printing techniques, such as silk-screen printing, flexographic printing, letterpress printing, intaglio printing, offset printing and the like.
Such a circumferential bearing is very stable and, moreover, provides sufficient space during silk-screen printing to receive a squeegee in the screen. Also, by using the same circumferential bearing for all types of printing techniques the same driving motors and driving control can be used for all types of printing techniques, which is very favorable from considerations of costs. During use of the printing machine according to the invention too, a considerable saving of the operational costs is effected because the change of printing technique is much leas time-consuming. In essence, only the cylindrical element needs to be exchanged, which, as a result of the circumferential bearing, can be done in a small amount of time. The driving means further remain untouched. The specific ink application rollers, squeegees and the like, belonging to a specific printing technique, for transferring the ink to the surface of the cylindrical element can be readily exchanged and are drivably connected to the driving means of the cylindrical element. The setting times can be considerably shortened by this exchange method.
As stated above, it is a frequently occurring drawback of the printing machines known from practice that when changing printing techniques in a specific module, if this is possible at all, the impression roller must be frequently exchanged as well. Apart from the expenditure of time, the exchange of the impression roller also produces a considerable amount of waste.
According to a further elaboration of the invention, the ink application means are bearing-mounted in a relevant printing module in a manner such that they are removable and positionable without it being necessary to remove the substrate web from the relevant printing module, the impression roller being provided with a flexible surface and being bearing-mounted for free rotation.
In the market, the prejudice existed that a hard impression roller was a requisite for flexographic printing and letterpress printing. Supposedly, the impression roller had to be hard in order to obtain the required printing sharpness and, moreover, to obtain a stable drive of the substrate web. In these printing techniques, the impression roller was actually also used as a substrate web driving roller. For a driving impression roller with a flexible surface the radius of the driving roller was believed to vary as a result of the tension in the substrate web. Supposedly, such a variable radius led to local speed differences of the substrate web, which gave considerable conveying problems and a poor printing quality. By using a non-driven impression roller with a flexible surface according to the above-described further elaboration of the invention, the conveying problems no longer occur anyway. Moreover, by using a hard impression roller with a flexible surface in the flexographic printing process or letterpress printing process, a very sharp printing quality can be obtained, in spite of the flexible surface of the impression roller. The printing quality may even be better than was hitherto conventional. For silk-screen printing a soft rubberized impression roller was conventional in connection with the fact that the silk-screen printing screen which replaces the printing roller used during flexographic printing is rather hard and the roughnesses in the substrate must therefore be taken up by the impression roller. Because the same impression roller can be used in any circumstances, it is no longer necessary to exchange the impression roller, and the substrate web can remain in position during exchange of the ink application means, which leads to a considerable saving of time.
According to another elaboration of the invention, a relevant printing module can be provided with a substrate web conveyor roller which is drivable with a controllable drive, which substrate web conveyor roller serves to convey the substrate web, the ink application means of the or each printing module being provided with their own drive with an independently controllable speed, the printing machine being provided with a control for controlling the rotational speed of the or each substrate web conveyor roller and the driving speed of the drive of the ink application means of the or each printing module.
This independent control of the conveying speed of the substrate web and the rotational speed of the ink application means renders it possible to bring the ink application means of all printing modules, and in particular the printing roller for flexographic or letterpress printing or silk-screen printing screen, into a desired rotative position. The different printing modules can therefore be brought into and kept in a desired starting position, so that the printing process can be started with a minimum of printing losses. Moreover, the printing rollers or screens can be prevented from wandering relative to each other, that is to say, the rotative positions of these elements can be prevented from moving relative to each other. The drive of these means is in fact positively controllable. Thus, an excellent printing quality can be guaranteed with a minimum loss of substrate web and printing ink and a minimum of setting time.